Iranian Foreign Minister Mohammad Javad Zarif speaks to the media after closed-door nuclear talks on Iran take place in Vienna, Austria, July 15, 2014.
AP

Kerry brought up the threat posed by ISIS, a common enemy to Iran and the U.S. which has seized territory on Iran's doorstep in neighboring Syria and Iraq.

Iran has military advisers in those countries and now the U.S. is publicly backing a role for the Islamic Republic in the conflict.

"The fact is there is a role for nearly every country in the world to play, including Iran," said Kerry.

The administration insists that it will not coordinate military operations or intelligence sharing with Iran, which is ruled by an elite group of Shiite Muslim clerics.

Doing so, they say, would anger U.S. allies Saudi Arabia and the United Arab Emirates, two Sunni-led countries which have privately offered to help the U.S. expand airstrikes from Iraq into Syria.

The coalition against ISIS being forged by the U.S. State Department now stands more than 50 countries. Brennan says Saudi Arabia is to host training camps for Syrian fighters, Jordan will share intelligence, Germany is providing weapons to Iraq and France is conducting its own airstrikes in Iraq.

In a string of television appearances on Sunday, U.N. Ambassador Samantha Power said some countries would also join the fight against ISIS on the Syrian side of the border, from the air, at least.

"We will not do the airstrikes alone if the president decides to do the airstrikes," Power said.

Some U.S. allies, however, with France included, are concerned that airstrikes in Syria would violate international law since the Assad government has not given permission.